1. Field of the Invention
The present invention generally relates to extended surface area tubing. The present invention also generally relates to a machine that produces textured surfaces on both inner and outer surfaces of a tube. The textured surfaces may be patterns of ribs and grooves formed in the inner and outer surfaces of the tube.
2. Description of the Related Art
A heat exchanger tube may be used in a process that transfers heat between a first fluid inside the heat exchanger tube and a second fluid outside of the heat exchanger tube. The efficiency of heat transfer between the first fluid and the second fluid may be a complicated function that depends on the characteristics of the fluids, on the characteristics of the heat exchanger tube, and on the characteristics of fluid movement relative to the heat exchanger tube. The term xe2x80x9cfluidxe2x80x9d refers to a liquid, a gas, or a combination of a liquid and a gas. A heat exchanger tube may also be used to transfer heat between a fluid and a solid. The solid may be located inside or outside of the tube.
Each end of a tube may be pointed. A pointed tube may have reduced diameter cylindrical portions at each end of the tube that transition to a larger diameter main body section of the tube. A pointed tube may facilitate attachment of the tube to support structures. The support structures may be tube sheets of a heat exchanger. Tube sheets may support several tubes within a shell of a tube-and-shell heat exchanger. Fluid that is directed past outside surfaces of tubes of a tube-and-shell heat exchanger may flow in a direction that is substantially coaxial to a longitudinal axis of the shell of the heat exchanger. Tubes having pointed ends may be easier to position and seal to support structures than are tubes that do not have pointed ends. U.S. Pat. No. 5,311,661, which issued to Zifferer and which is incorporated by reference as if fully set forth herein, describes an apparatus that may be used to form heat exchanger tubes having pointed ends.
It is desirable to maximize the heat transfer rate across a wall of a tube of a heat exchanger. Increasing the surface area of a tube may increase the heat transfer rate across the tube. Also, directing fluid flow past and through a tube in desired fluid flow patterns may increase the heat transfer rate across the tube.
One method of increasing the surface area of a tube is to attach fins to an outer surface of the tube. Fins may be attached to a tube after the tube is formed, or fins may be formed in the outer surface of the tube. Fins may be formed on the outer surface of a tube by a finning tool of a finning machine. A finning tool typically includes three or four disks mounted on an arbor. The disks form a spiraled flight of fins on an outer surface of a tube during use. The fins formed by a finning tool may have heights that are greater than about 30 mils (0.030 inches). Generally, the fins formed by a finning tool are oriented substantially perpendicular to the longitudinal axis of the tube. A small amount of skew from a true perpendicular orientation allows the finning tool to provide a driving force to the tube that moves the tube through the finning machine.
Fins may be oriented substantially perpendicular to a longitudinal axis of the tube, or the fins may be oriented substantially parallel to the longitudinal axis of the tube. Fins on an outer surface of a tube that are substantially perpendicular to a longitudinal axis of the tube may be used in heat transfer applications where fluid flow is directed substantially perpendicular to the longitudinal axis of the tube. Heat exchanger tubes of condensers and evaporators may be finned tubes wherein the fins are oriented substantially perpendicular to longitudinal axes of the tubes. Fins that are oriented substantially parallel to a longitudinal axis of a tube may be used in heat transfer applications where fluid flow is directed substantially coaxial to the longitudinal axis of the tube. Tubes having fins that are oriented substantially parallel to longitudinal axes of the tubes may be used in tube and shell heat exchangers.
Another method of increasing the surface area of a heat exchanger tube is to texture the inner surface of the tube. A knurling tool may be used to form a groove and rib pattern on an inner surface of a tube. The knurling tool may be placed within the tube. Force may be applied to an outer surface of the tube to press the inner surface of the tube against the knurling tool. Pressing the inner surface of the tube against the knurling tool forms a knurl pattern on the inner surface of the tube.
A finning tool and a knurling tool may be used in combination to form a tube that has a finned outer surface and a knurled inner surface. U.S. Pat. No. 4,886,830, which issued to Zohler and which is incorporated by reference as if fully set forth herein, describes a method of forming a tube that has a finned outer surface and a knurled inner surface.
An alternate method of texturing a tube is to form a desired pattern of ribs and grooves on surfaces of a flat metal plate. The plate may then be rolled into a cylindrical shape. A weld may be formed to join the ends of the plate together and form a tube. U.S. Pat. No. 5,388,329, which issued to Randlett et al., describes a method of manufacturing an extended surface heat exchanger tube using a rolled and welded metal plate.
A heat transfer rate across a tube may be increased by directing fluid flow in a desired flow pattern through and by the tube. A desired flow pattern may increase internal mixing of the fluid. A desired flow pattern may promote non-laminar fluid flow of one or both of the heat exchange fluids. In a straight, smooth-walled cylindrical tube, fluid may flow past or through the tube in a laminar flow pattern. Laminar fluid flow may develop a boundary layer at a wall of the heat exchanger tube. The boundary layer may inhibit heat transfer throughout the fluid. Non-laminar fluid flow may minimize the formation of a boundary layer and promote internal mixing of the fluid so that heat transfer takes place throughout the fluid.
One method that may be used to obtain a desired fluid flow pattern is to change the geometrical configuration of the surfaces of a heat exchanger tube. The geometrical configuration of the surfaces of a heat exchanger tube may be changed by texturing the surfaces of the tube. Texturing the surfaces of the tube may increase the heat transfer surface area of the tube and promote internal mixing of fluid that flows through or by the tube.
Inner and outer surfaces of a tube may be simultaneously textured with a texturing machine. The texturing machine may include an outer knurling device and an inner knurling device. The knurling devices may be used to form grooves in inner and outer surfaces of a tube. The depth of the grooves may be less than about 35 mils (0.035 inches), and are preferably less than about 25 mils. The depth of the grooves may be greater than about 4 mils. The grooves formed in the outer surface of the tube may have a different depth and a different pattern than the grooves formed in the inner surface of the tube. The grooves formed in the surfaces of the tube may increase the surface area of the tube, promote internal mixing of fluid that flows by or through the tube, and inhibit formation of stagnant areas of fluid adjacent to inner and outer surfaces of the tube. The grooves may be formed in a helical pattern about a longitudinal axis of the tube. The angles of the helical patterns formed in the inner and outer surfaces of the tube may be less than about 45xc2x0 relative to the longitudinal axis of the tube. Angle patterns that are less than about 45xc2x0 relative to the longitudinal axis of the tube may allow the tube to be used as a heat exchanger element wherein fluid flows by and through the tube in directions that are substantially coaxial with the longitudinal axis of the tube.
Texturing in an outer surface of a tube may be formed in a helical pattern by a texturing machine. An angle of the pattern relative to a longitudinal axis of the tube may be less than 90xc2x0, and is preferable less than about 45xc2x0. The angle of the pattern relative to a longitudinal axis of the tube may be greater than about 2xc2x0. Texturing in an inner surface of the tube may also be formed in a helical pattern. An angle of the inner tube surface pattern relative to a longitudinal axis of the tube may be less than about 90xc2x0, and may preferably be between about 5xc2x0 and 45xc2x0. The angle of the inner tube surface pattern relative to a longitudinal axis of the tube may preferably be about 30xc2x0.
An embodiment of a texturing machine may be used to form a texturing pattern in an outer surface of a tube that is oriented in an opposite direction to a texturing pattern formed in an inner surface of the tube. For example, a pattern formed in an outer surface of a tube may be a 20xc2x0 right-hand helical orientation of grooves, while a pattern formed in an inner surface of the tube may be a 30xc2x0 left-hand helical orientation of grooves. In an alternate embodiment, the angle pattern in the outer tube surface may be formed in a left-hand helical orientation, and the angle pattern in the inner tube surface may be formed in a right-hand helical orientation. The oppositely oriented patterns may cause the formation of a crosshatched pattern in the outer and inner surfaces of the tube. The crosshatched pattern may be a result of grooves being formed in the outer surface when ribs are formed on the inner surface. Similarly, grooves may be formed in the inner surface when ribs are formed on the outer surface. Embodiments of texturing machines may form helical patterns in tubing that are in the same orientation. For example, helical patterns in inner and outer tube surfaces may both be formed in right-hand helical orientations. Helical patterns in inner and outer tube surfaces may also both be formed in left-hand helical orientations.
An outer knurling device of a texturing machine may include one or more knurling tools. In an embodiment, the outer knurling device includes three knurling tools that are offset from each other by 120xc2x0. The outer knurling tools may be connected to drive mechanisms. When the drive mechanisms are engaged, the knurling tools rotate. The outer knurling device may also be coupled to a mechanism that brings the knurling tools into contact with a tube. When the knurling tools are brought into contact with a tube and when the drive mechanisms are engaged, the knurling tools rotate and form a helical pattern of grooves in an outer surface of the tube. The rotation of the knurling tools may drive the tube through the texturing machine.
In an embodiment, an angle of each outer knurling tool of a texturing machine may be adjustably positionable relative to a longitudinal axis of a tube positioned within the texturing machine. The outer knurling tools may be angled from about 0.50 to about 4.5xc2x0 in 0.5xc2x0 increments. The lower ends of the knurling tools may be positioned close to an exit end of the texturing machine. Each outer knurling tool may be set at the same angle. The set angle of the outer knurling tools may determine the feed rate of a tube through the texturing machine. For a tube that is made of a material that is difficult to work, e.g. titanium or cupro-nickel, a small set angle may be preferred. For a tube that is made of a material that is easy to work, e.g. copper, a larger set angle may be preferred so that there is a higher production rate of textured tubing from the texturing machine.
A tube may be positioned over an inner knurling device. The inner knurling device may be positioned beneath an outer knurling device of a texturing machine. The inner knurling device may be rotatively coupled to a mandrel. When a knurling tool or knurling tools of an outer knurling device are brought into contact with an outer surface of a tube, the outer knurling device may press an inner surface of the tube against the inner knurling device. When a drive mechanism or drive mechanisms of the knurling device are engaged to move the tube through the texturing machine, the inner knurling device forms texturing on the inner surface of the tube as the outer knurling device forms texturing on the outer surface of the tube.
A tube that is to be textured by a texturing machine may be placed over a mandrel of the machine so that a portion of a first end of the tube extends beyond the outer knurling device. The outer knurling device may be pressed against the tube to press an inner surface of the tube against the inner knurling device. A drive or drives may be engaged to move the tube through the machine so that the knurling devices form textured inner and outer tube surfaces. The drive or drives may be disengaged before the outer knurling device reaches a second end of the tube. Placing a portion of the first end of the tube beyond the outer knurling device and disengaging the knurling machine before reaching the second end of the tube leaves un-textured portions of tubing at each end of the tube. Un-textured portions of tube may allow the tube to be easily attached and sealed to support structures, such as tube sheets of a heat exchanger.
Each end of a textured tube may also be pointed by a pointing machine to promote easy attachment of the tube to support structures. To point an end of a tube, the end of the tube may be brought into contact with a tube-pointing die. The tube-pointing die may form a frustro-conical section and a cylindrical section having a reduced diameter at the end of the tube.
An advantage of a texturing machine is that the texturing machine may form a tube having inner and outer textured surfaces in a single operation. The textured surfaces may have increased surface area, and the textured surfaces may promote internal mixing of fluid that flows past the surfaces. Inner and outer textured surfaces may increase the effective heat transfer coefficient of the tube as compared to an un-textured tube of the same diameter.
Another advantage of the texturing machine is that a depth of grooves formed in inner and outer surfaces of a tube may be less than about 35 mils. The depth of the grooves may inhibit formation of stagnant fluid areas adjacent to the inner and outer surfaces of the tube while still promoting internal mixing of fluid flowing by or through the tube.
Another advantage of the texturing machine is that the texturing machine may be used to form texturing patterns having a variety of angle patterns and orientations. Different knurling devices may be installed in the texturing machine to form different patterns and different orientations. The angle of the patterns formed in the inner and outer tube surfaces may be less than about 90xc2x0 relative to a longitudinal axis of the tube. The angle of the patterns formed in the inner and outer tube surfaces may preferably be less than about 45xc2x0 relative to the longitudinal axis of the tube to promote efficient heat transfer across the tube when the flow of fluid by and through the tube is directed substantially coaxial to the longitudinal axis of the tube. The texturing machine may be used to form a helical pattern in an outer surface of a tube in a first direction that is opposite in orientation to a helical pattern formed in an inner surface of the tube. Oppositely oriented helical patterns may result in the formation of a crosshatched pattern in the inner and outer surfaces of the tube.
Another advantage of a texturing machine is that the angle of outer knurling tools relative to a longitudinal axis of a tube positioned within the machine may be adjustable. Adjusting the angle of the knurling tools allows a user to control throughput of tubing processed by the texturing machine. The throughput of the machine may be controlled to compensate for differences in hardness and workability of different types of tubing.
Another advantage of a texturing machine is that the texturing machine may leave un-textured portions at each end of the tube. The un-textured portions may allow the tube to be easily attached and sealed to support structures. Also, end portions of a textured tube may be pointed to allow the tube to be easily and conveniently attached and sealed to a support structure. A tube may be sealed to a support structure by a sealing method. The sealing method may be, but is not limited to, welding or application of sealant. Attaching a textured tube that has un-textured ends may be easier to accomplish than attaching a textured tube with textured ends because special procedures do not have to be implemented to ensure that a seal is formed adjacent to all of the grooves and ribs formed in the tube. Other advantages of a texturing machine may include that the texturing machine is sturdy, durable, simple, efficient, reliable and inexpensive; yet the machine is also easy to manufacture, install, maintain and use.